1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive sheet and a laminate of pressure-sensitive adhesive sheets. More particularly, it pertains to a pressure-sensitive adhesive sheet and a laminate composed of a plurality of said sheets superimposed on one another. Both the sheet and laminate are prevented from curling and are well suited for a home-delivery label, a commodity display label, a warning label and like label that are adhesively bonded to home-delivery cargoes, commodity packages, etc.
2. Description of the Related Arts
A label has heretofore been adhesively bonded to the surface of a home delivery cargo, a commodity package, etc. in order to display the kind, handling, etc. of the commodity. The display label such as a home-delivery label, a product display label and a warning label has heretofore been used in such a manner that a display label superimposed on a mount sheet, which is composed of release paper and is in the form of sheet or tape, is peeled off from the mount sheet in the form of sheet, or peeled off therefrom after cutting a mount sheet in the form of tape at prescribed positions.
However, such a label suffers from the disadvantage that the release paper remains as refuse, which increases waste materials in response to the amount of the release paper to be used and thereby brings about environmentally unfavorable results and besides that a worker is in danger of being injured at the time of cutting a mount sheet in the form of tape.
In order to overcome the above-mentioned disadvantage, Japanese Utility Model Registration No. 3,003,840 proposes a laminate composed of a plurality of pressure-sensitive adhesive sheets superimposed on one another, each of which comprises a substrate sheet, a pressure-sensitive adhesive layer formed by applying a pressure-sensitive adhesive onto one side of the substrate sheet, leaving the uncoated portion on the edges (periphery) thereof and a release layer formed on the other side of the substrate sheet, said pressure-sensitive adhesive layer and said release layer directly facing each other. The aforesaid pressure-sensitive adhesive sheet is advantageous in that the sheets can be peeled off one by one and adhesively bonded to an object in quite a simple manner with good workability and safety without causing waste materials such as release paper.
However, since the cured product from a conventional heat curing type silicone resin is used in the release layer of the pressure-sensitive adhesive sheet which constitutes the foregoing laminate and the silicone resin is cured at a high temperature of about 100 to 200.degree. C. in the production step of the pressure-sensitive adhesive sheet, the objective pressure-sensitive adhesive sheet is inevitably curled by the deformation of the substrate sheet due to heat as well as the expansion and contraction of the substrate sheet due to the release and absorption of moisture. In addition, a large-scale installation is required along with long working hours. Furthermore, most of the heat curing type silicone resins involve the problem of exerting adverse influence upon safety, sanitation and environment because of a large amount of a solvent used in the production step. FIG. 1 is an example showing a perspective view of a conventional pressure-sensitive adhesive sheet laminate in which an edge is curled.
On the other hand, UV and EB curing systems which have recently been developed employ ultraviolet rays (UV), and electron beam (EB), respectively and are characterized in that they enable the silicone resin to be cured under a solventless condition at a low temperature, thus contributing to low pollution, resources saving and labor saving and besides making it possible to produce high-performance coatings. Accordingly the above-mentioned systems have found wide use in a variety of fields such as coating materials, inks, adhesives, coating agents, resists, photosensitive printing boards and dental materials.